The present invention relates to a DCxe2x80x94DC converter, and more particularly, to a DCxe2x80x94DC converter in various types of electronic devices.
A DCxe2x80x94DC converter includes a step-down circuit, which generates a DC output voltage that is lower than a power supply voltage, and a step-up circuit, which generates a DC output voltage that is higher than the power supply voltage. The DCxe2x80x94DC converter further includes a control circuit for receiving a switching signal from an external device to switch the operational mode from a step-up operation to a step-down operation or from a step-down operation to a step-up operation. The DCxe2x80x94DC converter must have an external terminal to receive the switching signal. Accordingly, it is difficult to reduce the size of the DCxe2x80x94DC converter.
Referring to FIG. 1, a step-down DCxe2x80x94DC converter 100 includes a control circuit 1, which is formed on a single semiconductor integrated circuit substrate, and a step-down circuit 2, which includes a plurality of externally connected devices. The control circuit 1 controls the step-down circuit 2 to decrease an input voltage Vin and generate a DC output voltage Vo.
The control circuit 1 includes an error detection amplifier 3, which has a minus input terminal for receiving an output voltage Vo of the step-down circuit 2 and a plus input terminal for receiving a reference voltage VR1. The error detection amplifier 3 amplifies the differential voltage between the output voltage Vo and the reference voltage VR1 and provides the amplified signal to a first plus input terminal of a PWM comparator 4.
A duty setting signal DTC, which is a DC voltage, is provided to a second plus input terminal of the first PWM comparator 4. An oscillation circuit (not shown) provides a triangular wave signal VCT to a minus input terminal of the first PWM comparator 4.
The first PWM comparator 4 compares the output signal of the error detection amplifier 3 or the duty setting signal DTC, whichever one has the lower voltage, with the triangular wave signal VCT. In each cycle of the triangular wave signal, the first PWM comparator 4 generates a comparison output signal SG1 at a low level when the voltage of the triangular wave signal VCT is higher than that of the signal having the lower voltage. When the voltage of the triangular wave signal VCT is lower than that of the signal having the lower voltage, the first PWM comparator 4 generates the comparison output signal SG1 at a high level.
A voltage shift circuit 5 shifts the output signal of the error detection amplifier 3 to a high potential and provides the shifted signal to a plus input terminal of a second PWM comparator 6. A minus input terminal of the second PWM comparator 6 is provided with the triangular wave signal VCT. The second PWM comparator 6 compares the shifted signal and the triangular wave signal VCT. In each cycle of the triangular wave signal VCT, the second PWM comparator 6 generates a comparison signal SG2 at a low level when the voltage of the triangular wave signal VCT is higher than that of the shifted signal and generates the comparison signal at a high level when the voltage of the triangular wave signal VCT is lower than that of the shifted signal.
The comparison output signal SG1 of the first PWM comparator 4 is provided to a switch circuit 8a via an inverter circuit 7a and to a switch circuit 8b. The comparison output signal SG2 of the second PWM comparator 6 is provided to the switch circuit 8b via an inverter circuit 7b and to the switch circuit 8a. 
The switch circuits 8a, 8b are provided with a switching signal CH from an external device (not shown) via a terminal T. When the switching signal CH goes low, the switch circuit 8a functions to provide the output signal of the inverter circuit 7a to a drive circuit 9a, and the switch circuit 8b functions to provide the output signal of the inverter circuit 7b to a drive circuit 9b. 
The drive circuit 9a is operated by the power provided from a power supply VCC and the ground GND. The drive circuit 9b is operated by the power provided from a power supply VDD and the ground GND. Although the voltage of the power supply VCC is higher than that of the power supply VDD, the two voltages may be the same.
The drive circuit 9a provides its drive output signal to the gate of a p-channel MOS transistor Tr1 in the step-down circuit 2. The drive circuit 9b provides its drive output signal to the gate of an n-channel MOS transistor Tr2 in the step-up circuit 2. In response to the drive signals from the control circuit 1, the transistors Tr1, Tr2 are alternately activated.
The step-down circuit 2 includes the transistors Tr1, Tr2, a diode D1, a coil L1, and a capacitor C1. The alternate activation of the transistors Tr1, Tr2 in the step-down circuit 2 decreases the input voltage Vin and generates a step-down DC output voltage Vo.
In the step-down DCxe2x80x94DC converter 100, with reference to FIG. 3, the duty setting signal DTC is set at a voltage that is higher than the maximum voltage of the triangular wave signal VCT. When the DC output voltage Vo of the step-down circuit 2 decreases, the duty of high level in the comparison signals SG1, SG2, which are generated by the first and second PWM comparators 4, 6, increases. This lengthens the activated time of the transistor Tr1 and shortens the activated time of the transistor Tr2. As a result, the DC output voltage Vo of the step-down circuit 2 increases.
When the DC output voltage Vo of the step-down circuit 2 increases, the duty of high level in the comparison signals SG1, SG2, which are generated by the first and second PWM comparators 4, 6, decreases. This shortens the activated time of the transistor Tr1 and lengthens the activated time of the transistor Tr2. As a result, the DC output voltage Vo of the step-down circuit 2 decreases.
FIG. 2 is a schematic circuit diagram of a step-up DCxe2x80x94DC converter 200 that includes a step-up circuit 10. The step-up circuit 10 is driven by the control circuit 1 and increases an input voltage Vin to generate an output voltage Vo.
When the control circuit 1 is provided with the switching circuit CH at a high level, the drive circuit 9a is provided with the comparison output signal of the second PWM comparator 6 via the switch circuit 8a, and the drive circuit 9b is provided with the comparison output signal of the first PWM comparator 4 via the switch circuit 8b. 
The drive circuit 9a provides its drive output signal to the gate of a p-channel MOS transistor Tr3. The drive circuit 9b provides its drive output signal to the gate of an n-channel MOS transistor Tr4. In response to the drive signals from the control circuit 1, the transistors Tr3, Tr4 are alternately activated.
The step-up circuit 10 includes the transistors Tr3, Tr4, a diode D2, a coil L2, and a capacitor C2. The alternating activation of the transistors Tr3, Tr4 in the step-up circuit 10 increases the input voltage Vin and generates a step-up DC output voltage Vo.
In the step-up DCxe2x80x94DC converter 200, with reference to FIG. 4, the duty setting signal DTC is set at a voltage that is lower than the maximum voltage of the triangular wave signal VCT (more specifically, a voltage corresponding to about 70 percent of the amplitude of the triangular wave signal VCT).
When the DC output voltage Vo of the step-up circuit 10 decreases, the duty of high level in the comparison signals SG1, SG2, which are generated by the first and second PWM comparators 4, 6, increases. This shortens the activated time of the transistor Tr3 and lengthens the activated time of the transistor Tr4. As a result, the DC output voltage Vo of the step-up circuit 10 increases.
When the DC output voltage Vo of the step-up circuit 10 increases, the duty of high level in the comparison signals SG1, SG2, which are generated by the first and second PWM comparators 4, 6, decreases. This lengthens the activated time of the transistor Tr3 and shortens the activated time of the transistor Tr4. As a result, the DC output voltage Vo of the step-up circuit 2 decreases.
When an increase in the load of the step-up circuit 10 decreases the DC output voltage Vo, the duty of high level in the comparison output signal SG1 of the first PWM comparator 4 increases. This lengthens the activated time of the transistor Tr4. In this state, the voltage of the duty setting signal DTC is set at a value corresponding to about 70 percent of the amplitude of the triangular wave signal VCT. Thus, the transistor Tr4 does not remain activated. This prevents an overcurrent from damaging the transistor Tr4.
FIG. 5 is a schematic circuit diagram of a control unit 300, which includes two control circuits 80 and which is formed on a single semiconductor integrated circuit substrate.
A soft-start circuit 13 soft-starts a step-up circuit 10 or a step-down circuit 2 when a DCxe2x80x94DC converter is activated. When a load circuit of the DCxe2x80x94DC converter short-circuits, comparators 11a, 11b, AND circuits 12a, 12b, 12c, 12d, and an output short-circuit detection circuit 14 stop step-up or step-down operations. An oscillator 15 generates the triangular wave signal VCT, and a reference voltage generation circuit 16 generates reference voltages VR1, VR3.
In the control unit 300, which has a two channel configuration, each channel must have an external terminal to receive the switching signal CH. In this case, an external terminal having a total of 18 pins is necessary.
As described above, in the control circuits 1 and the control unit 300 of the DCxe2x80x94DC converters, the switching signal CH, which shifts the operational mode between the step-up and step-down operations, must be provided from an external device. Thus, an external terminal for receiving the switching signal CH is necessary. This increases the number of terminals in a semiconductor integrated circuit device that includes the control circuit 1 or the control unit 300. Therefore, it is difficult to produce a smaller device. Further, if a single semiconductor integrated device has a plurality of control circuits, this would further increase the number of external terminals, which receive the switching signal CH, and make it difficult to produce a smaller device.
It is an object of the present invention to provide a DCxe2x80x94DC converter having less external terminals.
To achieve the above object, the present invention provides a control circuit of a DCxe2x80x94DC converter that generates either one of a step-down control signal and a step-up control signal. The control circuit includes a switch circuit for outputting either one of the step-down control signal and the step-up control signal in response to a switching signal. A switching signal generation circuit is connected to the switch circuit to generate the switching signal using a duty setting signal, which controls either one of the step-down control signal and the step-up control signal.
A further perspective of the present invention is a control circuit of a DCxe2x80x94DC converter including a first PWM comparator for comparing an input signal, a duty setting signal, and a triangular wave signal to generate either one of a first step-down control signal and a first step-up control signal. A second PWM comparator compares the input signal and the triangular wave signal to generate either one of a second step-down control signal and a second step-up control signal. A switch circuit is connected to the first and second PWM comparators to output either the first and second step-down control signals or the first and second step-down control signals in response to a switching signal. A switching signal generation circuit is connected to the switch circuit to generate the switching signal using the duty setting signal.
A further perspective of the present invention is a DCxe2x80x94DC converter including a step-down circuit for decreasing an input voltage to generate a step-down output voltage or a step-up circuit for increasing the input voltage to generate a step-up output voltage. The DCxe2x80x94DC converter includes a control circuit connected to the step-down circuit or the step-up circuit for generating either one of a step-down control signal, which controls the step-down circuit, or a step-up control signal, which controls the step-up circuit. The control circuit includes a switch circuit for outputting either one of the step-down control signal and the step-up control signal in response to a switching signal. A switching signal generation circuit is connected to the switch circuit to generate the switching signal using a duty setting signal, which controls the duty of either one of the step-down control signal and the step-up control signal.
A further perspective of the present invention is a method for controlling a DCxe2x80x94DC converter including a step-down circuit or a step-up circuit. The method includes generating either one of a step-down control signal, which controls the step-down circuit, or a step-up control signal, which controls the step-up circuit. The method also includes generating a switching signal using a duty setting signal, which controls the duty of either one of the step-down control signal and the step-up control signal. The method further includes providing either one of the step-down control signal and the step-up control signal to the associated step-down circuit or step-up circuit in response to the switching signal.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.